The present invention relates to supplementary equipment for trawl winches and, more specifically a device for use on board a trawl vessel operating in autotrawl mode for dampening shock forces or alternating stresses on a pulling wire between the trawl vessel winch system and a bottom trawl or floating trawl, and a method making use of a compensating device equipped with hydraulic cylinder/piston means as damping members, sheave means operative with the cylinder and piston means, and hydraulic accumulator means connected to the cylinder means.
When a trawl vessel is trawling with a bottom trawl, the trawl board is dragged along the bottom. The force between the trawl board and the bottom may be 1-9 tons, depending on the size of the trawl gear and trawler. Due to the construction design of the trawl board, it is important for the steering of the trawl board and trawl that the board move at an even speed along the bottom during trawling, and that the force against the bottom be kept uniform.
Although, in the following, the invention is described specifically with a bottom trawl used as the trawl type, it shall be understood that the invention is equally applicable to trawling with a floating trawl.
In present day trawling on modern trawlers, an autotrawl system is used wherein the winch works against a "mooring" valve, during the actual trawling, in the attempt to maintain as uniform a force as possible in the trawl wire. This works well theoretically, but in practice many problems arise which interfere with the ability to maintain a uniform wire force. In practise, a preprogrammed wire length for the trawling controls the winch operation, such that wire length which has left the wire drum is kept within certain limits, thus yielding an almost constant force on the two wires used for the trawling operation.
Conditions that render it difficult to keep the wire force uniform are the following:
a) Substantial dynamic inertia in the trawl winch. This means that the stress variations created in the wires when the trawler vessel rides the waves cannot be properly averaged with the prior art trawl systems. PA1 b) The control system for the power regulating valve and the hydraulic system connected thereto is unable to control the wire force with sufficient precision. PA1 c) The trawler vessel in bad weather, when riding on large waves, may undergo very rapid variations in speed, resulting in failure of the winch system to attain a high enough capacity and speed to maintain equal force in the two trawl wires at all times. PA1 d) The trawl winches operate continuously under heavy stress by heaving in and paying out wire during trawling operations. The winches are controlled by the autotrawl system. This yields long operational periods for the winches, and the operation is also strongly changing from low to maximum speed and load. Such winch operation will cause heavy wear and tear, and experience has shown many cases of breakdown for trawl-winch systems operating in auto-trawl mode. Further, the prior art system has a continuous operational power demand of approximately 120 kW for operation in inclement weather. PA1 e) Considerable energy is used for trawling in the autotrawl mode when the weather is bad. PA1 f) The trawl wire is subjected to heavy wear and tear when trawling takes place in an autotrawl situation, as has hitherto been the case. Since the winches have substantial dynamic inertia, load or power peaks are reached which wear out the wire when it is payed out and heaved in during trawling in bad weather. In addition, there frequently arise situations in which the wire is arranged poorly on the drum, where the wire becomes crossed-over, and severe deformation and wear occur in autotrawling. PA1 g) In the prior art auto-trawl systems, the trawl speed varies between 6 and 2 knots when trawling under inclement weather conditions. The most efficient speed of the trawl when trawling for cod-fish is 5 knots.
Besides the disadvantages associated with failure to maintain a uniform wire force when trawling in inclement weather, there are other drawbacks, such as:
Thus, the efficiency of fishing would greatly improve if the trawl can be provided with an steady motion at the sea bottom. The more the smoothing of the trawl speed, the greater the fishing efficiency. If the trawl is moved stepwise or in intervals, it will not follow the bottom satisfactorily, and fish will escape above or below the trawl.